I guess the key question is this: If you are shooting raw, is HTP exactly equal to underexposing by 1 stop (so that you have 1 stop of extra headroom at the cost of losing dynamic range in shadows), or does it do something more advanced that cannot be simulated by appropriate post-processing? Different web sites seem to give conflicting answers regarding this part.
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Jukka SuomelaMay 18 '11 at 20:30

@Jukka According to @Dr.Elch answer, HTP doesn't put the sensor in a special mode or else. Just in-camera post-processing.
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AndresMay 19 '11 at 1:33

This page seems to confirm it; it claims that it has actually compared HTP ISO 200 and regular ISO 100, and the raw files are "identical". However, for example Canon EOS-1Ds Mark III marketing material claims that "Highlight Tone Priority expands the available range of capture in the highlights, yet it exacts no penalties in either shadow detail or other aspects of image capture." Could it be the case that HTP is implemented in a different manner in higher-end cameras?
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Jukka SuomelaMay 19 '11 at 6:29

Since most of the "special features" are developed for the high end products and then "downported" to the cheaper versions, I think that no, the Mark III doesn't have a different HTP version that a 550D, for example, could have.
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AndresMay 19 '11 at 14:16

5 Answers
5

Highlight tone priority is a camera mode that internally fiddles with exposure to preserve as much detail as possible in the "highlight range" of tones...the brightest tones in a photograph. It does this, however, at the cost of tones in the shadow range, as the ultimate effect is a shift of the histogram down towards the shadows. The cost of shadow tones is a bit less than the gain in highlight tones, however it is something to be aware of.

It should also be noted that HTP affects both RAW and JPEG images. This is in contrast to Automatic Lighting Optimizer, which applies an alternative tone curve when processing the sensor output for saving to JPEG. ALO only affects JPEG images, and has the tendency to compress dynamic range...thus costing you in your overall tonal range.

When you say HTP affects RAW files what do you mean? Do you mean that the sensed data stored in the RAW file is different, or that the RAW file comes with additional processing instructions on how the image should be "developed"? To put it another way, can I get an effect using HTP that I couldn't get by exposing differently and then developing my RAW file in the appropriate manner? Apologies if I have the terminology wrong here.
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fmarkMay 19 '11 at 11:58

To clarify, Thomas Knoll states that "As the raw level, HTP drops the ISO by 1 stop and underexposes by 1 stop." Is this the case, or is there something more going on?
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fmarkMay 19 '11 at 12:17

1

@fmark: HTP is achieved by the camera internally changing the exposure settings as you stated...so the output from the sensor affects every image, RAW or JPEG. This is in contrast with ALO, which is only applied during image processing when saving to JPEG...and therefor does not affect RAW files. You could get the HTP effect by raw processing...however, if you BLOW your highlights, no amount of post-processing will recover them...they are clipped. The benefit of HTP is highlights are preserved in-camera...which reduces the chances of you accidentally clipping them.
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jrista♦May 19 '11 at 20:07

This is one of the few image-enhancement settings that is extremely useful. It is in the custom menu because once you set it, you leave it and do not fiddle with it between shots.

When enabled, your camera will preserve more details in the highlights at the expense of some details in the shadows. If your style is to expose for the highlights, then it will greatly help. If you do not know what your style is, just turn it on, you'll thank me later ;)

NOTE: There is another semi-related setting called something like 'Lighting Optimizer', make sure that one is OFF since what it mostly does is make your photos look dull with less contrast and adds noise to shadow areas.

You do not lose details in the shadows, however, because the shadows are pushed by 1 stop you effectively always use at least ISO200 in the shadows and you might have just a little more noise.
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douweAug 28 '12 at 12:18

Highlight Tone Priority (HTP)
All cameras have a fixed dynamic range, from shadow
to highlight, that they can capture. HTP shifts some of
the available dynamic range from the mid-tones to the
highlights to produce smoother tones, with more detail
in bright areas. This helps prevent JPEG images with
overexposed highlights that can’t be recovered. HTP is
also useful to RAW shooters who process their images
with Canon’s DPP software. Most third-party RAW
processing software will not recognize Highlight Tone
Priority.
When the camera is set to HTP, the lowest available
ISO will be 200. The HTP setting will be indicated by a D+
symbol in the LCD display. Avoid using HTP in low light or
when shooting subjects with heavy shadows because it
may cause more noise to appear in those areas.

furthermore it says about automatic lighting optimization:

Auto Lighting Optimizer (ALO)
ALO performs in-camera processing to help preserve
shadow detail in high-contrast scenes. It also adds a
modest boost in contrast to low-contrast scenes. The
amount of adjustment can be set to Off (0), Low (1),
Standard (2) or Strong (3). ALO can be useful for JPEG
shooters working on a deadline and who don’t have time
to manually adjust the contrast of each photo.
Recent EOS models apply ALO at the Standard level
by default, even when set to Manual exposure mode.
Users who desire full control over shadow and highlight
values may therefore wish to turn ALO off.

Uhmmm, seems that HTP does nothing special to the sensor. According to this description, Lightroom could achieve the same results.
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AndresMay 19 '11 at 1:32

Well it seems to restrict your ISO range, but apart from that it is nothing that can't be achieved otherwise. I have seen lots of video folks that are dealing with ALO and HTP when using their canon DSLRs since here these changes can have a higher impact on the results obviously.
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Dr.ElchMay 19 '11 at 1:37

@Andres: The benefit of HTP is that it can help prevent you from capturing a photo with BLOWN highlights...or highlights that are unrecoverable even with Lightroom.
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jrista♦May 19 '11 at 2:02

Highlight priority underexposes by one stop by lowering the ISO, for this reason ISO100 cannot be selected with highlight priority. In post-process the camera compensates for the underexposure, except for the highlights

As a RAW shooter you can archieve the same result by underexposing all your shots with 1 stop and then lift everything but the highlights.

Some photographers always underexpose by 1 stop because sensors lose information in the highlights much easier then in the shadows. Film users also did this quite often by exposing and developing ISO 200 film as if it were ISO 100. Because of the characteristics of film the highlights where then actually automatically compressed and preserved.